The initially described negative pressure-type booster device includes a bulkhead member constituting a power piston in a booster shell. The bulkhead member is obtained by combining a pressure plate and a diaphragm, and the bulkhead member partitions the interior of the booster shell into a constant-pressure chamber and a variable-pressure chamber.
The constant-pressure chamber is connected to an intake manifold of an engine, and the variable-pressure chamber is connected to an atmospheric-air intake path. The power piston is configured such that a valve piston including a negative-pressure valve and an atmospheric-pressure valve which are disposed therein is attached to a central part of the bulkhead member.
In the negative pressure-type booster device, the negative-pressure valve is closed by a braking operation to cut a communication between the constant-pressure chamber and the variable-pressure chamber. Thereafter, the atmospheric-pressure valve opens to take atmospheric air into the variable-pressure chamber through the atmospheric-air intake path. In this manner, a pressure difference occurs between the constant-pressure chamber and the variable-pressure chamber, the pressure difference advances the power piston, and force amplified depending on braking operation force activates the master cylinder. Since the activation mechanism is well known, a detailed description thereof will be omitted.
The booster shell of the negative pressure-type booster device is configured by a combination of a front shell and a rear shell, and the master cylinder is fastened to the front shell by using two tie rods.
In the front shell, a through hole in which an input side of the master cylinder is inserted is formed at the center, and tie-rod holes through which the tie rods are inserted are formed around the through hole. The number of the tie-rod holes formed is two, and these two holes are disposed at respectively 180°-rotated positions with the through hole interposed therebetween.
The two tie-rod holes are also formed in the rear shell. In the rear shell, a bolt hole in which an attaching bolt is inserted is also additionally formed at a position turning from an installation point of each of the tie-rod holes at approximately 90 degrees in a circumferential direction.
An example of the booster shell obtained by combining the front shell and the rear shell in which the tie-rod holes are formed is described in, for example, the following Patent Literature 1. In a negative-pressure booster disclosed in the literature, reinforcement ribs are formed around the tie-rod holes in the front shell and the rear shell to reduce the thicknesses of the shells.
The reinforcement ribs, in each of the front shell and the rear shell, are obtained by combining a rib radially extending from the center of each of the shells, a circular rib disposed on a circle coaxial with the center of each of the shells, and a rib surrounding the opening of each of the tie-rod holes. The circular rib includes a plurality of ribs having different diameters and coaxially disposed.